Many types of electric motors incorporate an electronic commutator which adjusts the winding excitation according to the instantaneous position of the rotor. The position of the rotor may be determined mechanically, but is advantageously deduced from the motor's current and voltage waveforms. Mechanical determination can involve complex and expensive equipment, and may detrimentally affect the rotor's inertia and friction characteristics.
A paper entitled "Application of stochastic filtering to a permanent magnet synchronous motor-drive system without electro-mechanical sensors", by Dhaouadi, R. et al. (Proc. ICEM, Boston, 1990) discloses a method of determining rotor position which involves detecting electrical characteristics (phase voltages and currents) of the motor at given instants and estimating rotor position from these characteristics using an extended Kalman filter algorithm. An initial estimate of rotor position is provided by solving the mechanical equation of motion for the rotor. Phase flux linkage, the value for which is utilised in the determination of rotor position, is modelled as being linearly dependent on phase current.
A paper entitled "A state observer for variable reluctance motors: analysis and experiments", by Lumsdaine, A. et al. (Proc. ASILOMAR Conference on Circuits, Systems and Computers, Pacific Grove, 1985) discloses a similar method in which rotor position is estimated from the electrical characteristics using a state observer model.
The disadvantages with these techniques are that they are limited to specific types of electric motor and that they are limited to specific operating condition ranges.
The present invention seeks to solve these problems. In its various aspects, the invention provides techniques which can be applied to virtually any type of motor having phase flux linkage varying with rotor position. Such motor types include switched and variable reluctance motors, hybrid stepping motors, permanent magnet synchronous motors, brushless alternating and direct current motors, and sinusoidal and trapezoidal permanent magnet motors. In particular, the invention can be applied to any motor which is controlled in dependence on rotor position (so called "rotor position switched drives"). Other arrangements are described below.
International Patent Application No. PCT/SE90/00498 describes an arrangement in which magnetic flux in a motor is compared with tabulated values of a non linear function of magnetic flux with respect to current.
IEEE Transactions on Industry Applications Vol. 27 No. 5 pages 1005-1011 describes a controller for use to control motor speed without the need for a sensor. Two line voltages and stator currents are employed to compute a flux linkage space vector. An indication of angular velocity is then computed using the phase angle of the vector and the rate of change of flux linkage.
U.S. Pat. No. 5,107,195 describes an apparatus and method which simultaneously senses phase current and phase flux, determines the reluctance of the stator-to-rotor air gap and evaluates the position of the rotor phase with respect to the stator pole tips.